1. Field of the Invention (Technical Field)
The invention relates to displays and more particularly to dual mode transmissive display backlighting of color displays for use in a night mode such as in conjunction with Night Vision Goggles (NVG).
2. Background Art
A design of a Liquid Crystal Display (LCD) was needed with a backlight with two different modes of operation with different colors for each mode. An additional requirement was that one of the modes, the night mode, be compatible with Night Vision Imaging System (NVIS) requirements of MIL-L-85762A. This mode, NVIS mode, was also required to have a particular color as defined in MIL-L-85762A as NVIS Green A. The other mode of operation, day mode, needed to provide full daylight readability of the display in an avionic high ambient illumination condition, and was required to be white. The backlight illumination source was selected to be white LEDs for both modes of operation. The particular problem, which was encountered with the initial design, was associated with the phosphorescence of the day mode LEDs when they were illuminated with radiant energy from the NVIS filtered night mode LEDs. The day mode LEDs absorbed NVIS compatible illumination and re-radiated non-compliant energy. This phosphorescence caused the display to fail the NVIS requirements of MIL-L-85762A.
A typical dual mode backlight would provide an NVIS filter that acts only in the NVIS mode lighting source, which would not prevent the phosphorescence of the day mode lighting source from creating NVIS problems through phosphorescence. Other day mode lighting sources (incandescent lamps, fluorescent lamps, etc.) do not typically exhibit phosphorescence in the NVIS region of interest (i.e., absorption in visible spectrum, 380–780 nm, and re-emission in the NVIS spectrum, 570–930 nm). Placing a conventional NVIS filter over the day mode lighting source would severely limit the efficiency of the day mode lighting system, which would require the use of more LEDs and more power to achieve day mode requirements of legibility in high ambient illumination conditions.
There are patents for dual-mode backlights/displays, which are NVIS compatible. However, the previous designs use different illumination sources, and do not require the optical filtering techniques required for the present invention. The introduction of white LEDs caused the issue to arise. LEDs were selected for this particular design for several reasons. They have improved reliability relative to other types of lighting sources (i.e., incandescent lamps, fluorescent lamps), and they perform extremely well over a very wide operating temperature which is advantageous for demanding environmental conditions as are typically encountered with avionic or automotive display products. LEDs are also very easy to drive electrically and to dim as is required for backlights in avionic applications. Typically, if a material phosphoresces and causes problems with NVIS radiance, the material is removed from the design or it is placed behind the NVIS filter. Due to the use of the white LEDs for the day mode backlight design, removal is not possible, and filtering with a typical NVIS filter would significantly inhibit meeting the day mode requirements, thereby eliminating one of the natural advantages of a dual mode backlighting system.
There are several prior art devices that disclose dual mode backlighting systems. These include U.S. Pat. No. 5,986,730, for an LCD that is both transmissive and reflective and U.S. Pat. Nos. 6,285,425 and 6,285,426, which involve using a reflector to allow a display to be illuminated from ambient light. Some of the prior art devices that use an NVIS filter for filtering the NVIS light source include U.S. Pat. No. 5,982,090, which discloses a special kind of flat fluorescent lamp used in LCD backlighting and U.S. Pat. No. 6,039,451, which teaches the use of a transflector that is used to enhance day mode luminance. U.S. Pat. No. 6,419,372 is a dual mode LED backlighting system that uses a light guide to distribute the two different LED lighting systems. This system includes NVIS filtering that is only applicable to the night mode LEDs and does not address the issue of phosphorescence, which will result from the NVIS illumination radiating onto the day mode LEDs.
None of the prior art devices teach or imply the use of a filter placed over the inactive or “day mode” light source as presently described in the present application.